Technical Briefs

Optical Strain Measurement Techniques to Assist in Life Monitoring of Power Plant Components

[+] Author and Article Information
Andrew Morris

E.ON Engineering, Technology Centre, Ratcliffe-on-Soar, Nottingham NG11 0EE, UK

Chris Maharaj, Miltiadis Kourmpetis, Amit Puri, John Dear

Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK

Ian Dear

School of Engineering and Design, Brunel University, Uxbridge, Middlesex UB8 3PH, UK

J. Pressure Vessel Technol 131(2), 024502 (Jan 21, 2009) (8 pages) doi:10.1115/1.3062935 History: Received September 06, 2007; Revised March 28, 2008; Published January 21, 2009

Sensors for monitoring creep strain in high-pressure steam pipes and other power plant components are subjected to very demanding environmental and operational conditions. It is important that the sensors are of a rugged design and that measurement can be made that only relates to creep movements in power plant components. The E.ON UK auto-reference creep management and control (ARCMAC) optical strain gauges have been designed to have this capability. These optical strain gauges are installed across sections of welded steam pipe and other plant components in locations that provide the best monitoring points to reveal the early onset of failure processes. Reported in this paper are recent developments to improve optical creep strain measurement to achieve a 65 microstrain accuracy level with an error of less than 10%. Also reported are trials of combining optical strain gauges with digital image correlation (DIC) to obtain detailed information of the creep strain distribution around the gauges. The DIC data for known defect geometries have been validated with finite element analysis.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 4

FE and DIC of CMV steel tensile specimen with rear surface defect: (a) von Mises stress (megapascals) shown on the rear surface with inset view showing the von Mises stress (megapascals) on the front surface, (b) εy strain field from FE and DIC for maximum load (60 kN), (c) εx strain field from FE and DIC for maximum load (60 kN), (d) εy strain field from FE and DIC at a load of 53 kN, and (e) εx strain field from FE and DIC at a load of 53 kN (white dotted line shows the outline of defect on the rear surface)

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Figure 5

DIC of different weld geometries: (a) half of tensile specimen with single-V and double-V geometries for MMA welding (dimensions in millimeters), (b) εy for a single-V weld and a double-V weld at 72% and 74%, respectively, of the fracture load (failure loads were 145 kN for single-V and 135 kN for double-V geometries), and (c) εy for a single-V weld and a double-V weld just before complete fracture with Vickers hardness measurements

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Figure 3

Steel tensile specimen with hidden defect: (a) specimen with rear surface defect and optical strain gauge, (b) image of specimen showing speckle pattern and optical gauge on the right-hand side, (c) DIC strain plots (εy and εx) of the front surface of steel tensile specimen with hidden defect on the rear surface at 85% of maximum load (17 kN) and at maximum load (20 kN) (white dotted line shows the outline of defect on the rear surface)

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Figure 2

Percentage error ((ε−εECR)/εECR×100) in the measurement of optical strain (ε) compared with absolute NPL ECR strain (εECR) for luminescent strip light source (black diamonds) and LED light source (gray squares) for the normal operating position and for axial displacements of 1 mm, 2 mm, and 4 mm from this normal position

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Figure 1

Optical strain measurement system: (a) camera unit, (b) uniaxial gauge, (c) defining ratio (B/A) for gauge, (d) biaxial gauge image, and (e) exploded view of biaxial gauge installation



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